SUMMARY1. Nutrient and fish manipulations in mesocosms were carried out on food-web interactions in a Mediterranean shallow lake in south-east Spain. Nutrients controlled biomass of phytoplankton and periphyton, while zooplankton, regulated by planktivorous fish, influenced the relative percentages of the dominant phytoplankton species. 2. Phytoplankton species diversity decreased with increasing nutrient concentration and planktivorous fish density. Cyanobacteria grew well in both turbid and clear-water states. 3. Planktivorous fish increased concentrations of soluble reactive phosphorus (SRP). Larger zooplankters (mostly Ceriodaphnia and copepods) were significantly reduced when fish were present, whereas rotifers increased, after fish removal of cyclopoid predators and other filter feeders (cladocerans, nauplii). The greatest biomass and diversity of zooplankton was found at intermediate nutrient levels, in mesocosms without fish and in the presence of macrophytes. 4. Water level decrease improved underwater light conditions and favoured macrophyte persistence. Submerged macrophytes (Chara spp.) outcompeted algae up to an experimental nutrient loading equivalent to added concentrations of 0.06 mg L )1 PO 4 -P and 0.6 mg L )1 NO 3 -N, above which an exponential increase in periphyton biomass and algal turbidity caused characean biomass to decline. 5. Declining water levels during summer favoured plant-associated rotifer species and chroococcal cyanobacteria. High densities of chroococcal cyanobacteria were related to intermediate nutrient enrichment and the presence of small zooplankton taxa, while filamentous cyanobacteria were relatively more abundant in fishless mesocosms, in which Crustacea were more abundant, and favoured by dim underwater light. 6. Benthic macroinvertebrates increased significantly at intermediate nutrient levels but there was no relationship with planktivorous fish density. 7. The thresholds of nutrient loading and in-lake P required to avoid a turbid state and maintain submerged macrophytes were lower than those reported from temperate shallow lakes. Mediterranean shallow lakes may remain turbid with little control of zooplankton on algal biomass, as observed in tropical and subtropical lakes. Nutrient loading control and macrophyte conservation appear to be especially important in these systems to maintain high water quality.

1. Results are analysed from 11 experiments in which effects of fish addition and nutrient loading on shallow lakes were studied in mesocosms. The experiments, five in 1998, six in 1999, were carried out in six lakes, distributed from Finland to southern Spain, according to a standard protocol. 2. Effects of the treatments on 29 standard chemical, phytoplankton and zooplankton variables are examined to assess the relative importance of bottom-up (nutrient enrichment) and top-down (fish predation) effects. For each year, the experiments in different locations are treated as replicates in a meta-analysis. Results of individual experiments are then compared in terms of the patterns of significant influences of nutrient addition and fish predation with these overall results (the baseline), and between years in the same location. 3. The overall meta-analysis gave consistent results across the 2 years, with nutrient loading influencing all of the chemical variables, and on average 31% of primary producer and 39% of zooplankton variables. In contrast, fish influenced none of the chemical variables, 11% of the primary producer and 44% of the zooplankton variables. Nutrient effects on the system were thus about three times greater than fish effects, although fish effects were not inconsiderable. 4. The relative importance of nutrients and fish in individual experiments often differed between years at the same location and effects deviated to varying degrees from the baseline. These deviations were treated as measures of consistency (predictability) of conclusions in repeat experiments. Consistency increased southwards and this is interpreted as a consequence of more variable annual weather northwards. 5. The influence of nutrient loading was greater southwards and this was probably manifested through naturally greater annual macrophyte abundance in warmer locations in consequence of the longer plant growing-season. There was no trend in the relative importance of fish effects with latitude but this may partly be an artefact of the simple fish Correspondence: Brian Moss,

Summary 1. In view of the paucity of data on the response of warm shallow lakes to reductions in nutrient loading, this paper presents a long‐term limnological data set to document changes in the food‐web of a shallow Mediterranean lake (Lake Albufera, Valencia, Spain) that has experienced reductions in phosphorus (P) (77%) and nitrogen (N) (24%) loading following sewage diversion. 2. Nine years after sewage diversion, P concentration in the lake was reduced by 30% but remained high (TP = 0.34 mg L−1), although the mean water retention time in the lake was only 0.1 years. Nitrate concentrations did not significantly change, probably because the lake continued to receive untreated effluents from ricefields. 3. Chlorophyll a concentration was reduced by half (annual mean of 180 μg L−1). Cyanobacteria abundance remained high but its composition changed towards smaller species, both filamentous and chroococcal forms. 4. Cladocera abundance increased and reached peaks twice a year (December to March and July to September). After nutrient reduction, short‐term clear‐water phases (up to 5 weeks) occurred during February to March in several years, concomitant with annual flushing of the lake and lower fish densities. The abundance of Cladocera in winter contrasted with the spring peaks observed in northern restored shallow lakes. The zooplankton to phytoplankton biomass ratio remained lower than in northern temperate shallow lakes, probably because of fish predation on zooplankton. 5. Improvement of the water quality of Lake Albufera remained insufficient to counteract littoral reed regression or improve underwater light allowing submerged plants re‐colonise the lake. 6. Sewage diversion from Lake Albufera impacted the food web through the plankton, but higher trophic levels, such as fish and waterfowl, were affected to a lesser degree. Although the fish species present in the lake are mainly omnivorous, long‐term data on commercial fish captures indicated that fish communities changed in response to nutrient level and trophic structure as has been observed in restored shallow lakes at northern latitudes. 7. Phosphorus concentrations produced similar phytoplankton biomass in Lake Albufera as in more northern shallow lakes with abundant planktivorous fish and small zooplankton. However, in Lake Albufera, high average concentrations were maintained throughout the year. Overall, results suggest that nutrient control may be a greater priority in eutrophicated warm shallow lakes than in similar lakes at higher latitudes.

Dispersal is a major organising force in metacommunities, which may facilitate compositional responses of local communities to environmental change and affect ecosystem function. Organism groups differ widely in their dispersal abilities and their communities are therefore expected to have different adaptive abilities. In mesocosms, we studied the simultaneous compositional response of three plankton communities (zoo-, phyto- and bacterioplankton) to a primary productivity gradient and evaluated how this response was mediated by dispersal intensity. Dispersal enhanced responses in all three planktonic groups, which also affected ecosystem functioning. Yet, variation partitioning analyses indicated that responses in phytoplankton and bacterial communities were not only controlled by dispersal directly but also indirectly through complex trophic interactions. Our results indicate that metacommunity patterns emerging from dispersal can cascade through the food web and generate patterns of apparent dispersal limitation in organisms at other trophic levels.

We studied the diet of the eastern mosquitofish Gambusia holbrooki with in situ experimental mesocosms located in a shallow lake. Different nutrient concentrations (phosphorus and nitrogen) and fish population densities were tested. Our results confirm that it is a planktivorous species, with also a great ingestion of algae and detritus. Nutrient fertilization caused almost no changes in this species feeding behavior, but larger mosquitofish stocks induced a shift to zooplanktivory and a decline in detritivory. When macrophytes were present, the predation effect focused on zooplankton and plantassociated animals, otherwise predation on bottom macroinvertebrates increased. Females preyed upon almost all groups more intensely, including detritus. Males and juveniles did not overlap diet, the former being more selective on ostracods, while juveniles consumed detritus, rotifers and cladoceran. Our data support the idea that mosquitofish can cause important top-down effects in shallow lakes under a wide variety of ecological conditions, being an important zooplanktivore in both turbid and plantdominated shallow lakes especially in the Mediterranean zone, where high temperatures and absence of piscivores promote maintenance of its populations during the whole year.

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Summary 1. Shallow lake ecosystems are normally dominated by submerged and emergent plants. Biological stabilising mechanisms help preserve this dominance. The systems may switch to dominance by phytoplankton, however, with loss of submerged plants. This process usually takes place against a background of increasing nutrient loadings but also requires additional switch mechanisms, which damage the plants or interfere with their stabilising mechanisms. 2. The extent to which the details or even major features of this general model may change with geographical location are not clear. Manipulation of the fish community (biomanipulation) has often been used to clear the water of algae and restore the aquatic plants in northerly locations, but it is again not clear whether this is equally appropriate at lower latitudes. 3. Eleven parallel experiments (collectively the International Mesocosm Experiment, IME) were carried out in six lakes in Finland, Sweden, England, the Netherlands and Spain in 1998 and 1999 to investigate the between‐year and large‐scale spatial variation in relationships between nutrient loading and zooplanktivorous fish on submerged plant and plankton communities in shallow lakes. 4. Comparability of experiments in different locations was achieved to a high degree. Cross‐laboratory comparisons of chemical analyses revealed some systematic differences between laboratories. These are unlikely to lead to major misinterpretations. 5. Nutrient addition, overall, had its greatest effect on water chemistry then substantial effects on phytoplankton and zooplankton. Fish addition had its major effect on zooplankton and did not systematically change the water chemistry. There was no trend in the relative importance of fish effects with latitude, but nutrient addition affected more variables with decreasing latitude. 6. The relative importance of top‐down and bottom‐up influences on the plankton differed in different locations and between years at the same location. The outcome of the experiments in different years was more predictable with decreasing latitude and this was attributed to more variable weather at higher latitudes that created more variable starting conditions for the experiments.

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